1. Field of the Invention
The present invention relates to a display control circuit for displaying external input image data, a liquid crystal display device including the same, and a display control method. More specifically, the present invention relates to a display control circuit correcting input image data obtained by use of data in an immediately preceding frame, a liquid crystal display device including the same, and a display control method.
2. Description of the Background Art
A liquid crystal particle for use in a liquid crystal display device has an optical response time which is variable. The liquid crystal particle can not ensure a quick response and typically requires several tens of milliseconds. For this reason, it is preferable that this liquid crystal display device performs the following operation. For example, it is assumed herein that a display gray scale level is in a range from 0 to 255 in the liquid crystal display device. In order to display an image at the display gray scale level of 100, even when the display gray scale level is 0 in a preceding vertical display period (hereinafter, referred to as a “frame”), the liquid crystal display device changes the display gray scale level from 0 to 100 in this frame.
As described above, however, the liquid crystal particle can not ensure such a response that the display gray scale level is changed to 100 immediately. In actual, the display gray scale level achieves 100 after a lapse of several tens of milliseconds. During a period until the display gray scale level achieves 100, consequently, the liquid crystal display device continuously displays the image at the display gray scale level other than 100 (e.g., a gray scale level which is lower than 100 in a display device of a normally black type), resulting in degradation of display quality.
In order to solve the problem which causes the degradation of the display quality, that is, the problem about the response speed dealing with the change in gray scale level, conventionally, various countermeasures have been taken for the liquid crystal display device. For example, Japanese Patent Laid-Open Publication No. 04-288589 discloses a liquid crystal display device including an image memory that holds an input image signal in one frame. This liquid crystal display device detects a level variation between an input image signal in a preceding frame, which is held by the image memory, and an input image signal in a current frame. For example, this detection of the level variation corresponds to detection of the above-described change of the gray scale level from 0 to 100. Upon detection of such a level variation, an input image is subjected to high-frequency emphasis filtering, so that the liquid crystal display device can be improved in response speed. Hereinafter, this conventional configuration is referred to as a first conventional example.
Moreover, International Publication No. WO 03/098588 discloses a configuration capable of improving a response speed of a liquid crystal display device which is inferior in response performance to the liquid crystal display device described above. This configuration is different from the configuration in the first conventional example. More specifically, the liquid crystal display device obtains, based on input image data in a preceding frame, a predicted value of a display gray scale level of an image to be displayed actually thereon after a lapse of this frame, and an image memory of the liquid crystal display device holds this predicted value rather than an input image. Hereinafter, this conventional configuration is referred to as a second conventional example. According to this configuration, in order to improve the response speed, a liquid crystal particle is applied with a voltage corresponding to a value which varies largely as compared with the predicted value. For this reason, this drive method is also referred to as an overshoot drive method.
The first conventional example and the second conventional example are different from each other in the details of image data in a preceding frame, the image data being used for correcting an input image signal. However, the first conventional example and the second conventional example are equal to each other in the point that the liquid crystal display device includes the image memory holding image data in one frame. In these configurations, the liquid crystal display device includes a display panel having a display size of WXGA (1,366×768) and a display gray scale level consisting of 8-bit R data, 8-bit G data and 8-bit B data. In the liquid crystal display device, image data to be stored in the image memory has a size of about 25,000,000 (1,366×768×8×3) bits.
In order to avoid this disadvantage, U.S. Patent Publication No. 2005/0200631 discloses the following configuration. That is, image data is stored in an image memory while being compressed appropriately by a block truncation coding method so as to have a small data size. Hereinafter, this coding method is simply referred to as a BTC method. However, detailed description of the compression method described in U.S. Patent Publication No. 2005/0200631 will not be given here. Basically, the compression is performed in an irreversible manner by three methods: (1) reduction of the number of bits, (2) color space conversion and down-sampling, and (3) reduction of spatial redundancy. The compression by the BTC method corresponds to the above-mentioned method (3). This configuration allows reduction in size of the image memory and reduction in manufacturing cost. Hereinafter, this conventional configuration is referred to as a third conventional example.
The third conventional example adopts two types of codes obtained by the BTC method, that is, a low compression code containing a variable length portion (hereinafter, referred to as an LBTC (Low-compression-ratio BTC)) and a high compression code serving as a fixed length portion (hereinafter, referred to as an HBTC (High-compression-ratio BTC)). This HBTC is a mean value obtained when an input image is divided into a plurality of blocks each consisting of vertical two pixels and horizontal two pixels. Accordingly, in a case where the respective pieces of data of the four pixels contained in one block are largely different in gray scale level from each other and an HBTC obtained by compression of these pieces of data is used as a predicted value of a display gray scale level in an immediately subsequent frame for the overshoot drive described above, this predicted display gray scale level occasionally differs from an actual display gray scale level. This difference is canceled normally in the subsequent frame, but is left occasionally depending on a status of the change in data. With regard to a certain pixel in one block, for example, in a case where a gray scale level largely changes from 0 to 255 and then returns to 0 for each frame, but a mean value of the gray scale levels of the four pixels does not vary largely, the (input) gray scale level of the relevant pixel actually returns to 0, but the predicted value of the display gray scale level does not return to 0 occasionally. Such a difference is left as noise in a form of an after-image (hereinafter, simply referred to as “after-image noise”) in the displayed image, resulting in degradation of display quality.